feat(hh): P3b — multipass lifecycle + PTY resize sync

- sbx.rs: prepare()/teardown() — multipass launch (idempotent, reuses an
  existing instance) on /sbx launch multipass, delete --purge on stop;
  Backend::default_image() per backend (24.04 / ubuntu:24.04).
- app.rs: async non-freezing launch — prepare runs in spawn_blocking and the
  sandbox handle is handed to the run loop via a channel, so a ~30s multipass
  VM boot never freezes the UI (status: "summoning…"). Sandbox is sized to the
  actual pane (not fixed 24x80); broker resizes the PTY and broadcasts
  {"_sbx":"resize"} on terminal-size changes; clients set their vt100 size to
  match. Teardown on /sbx stop and on exit.
- net.rs: parse status rows/cols + resize frames.

Verified: cargo test (4 pass), clean build, live local sandbox via the async
path with dynamic full-width sizing. multipass 1.16.3 present; VM-boot path
implemented (live VM verify is slow, runs the same async flow).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hh/src/sbx.rs

@@ -9,6 +9,7 @@
 use anyhow::{Context, Result};
 use portable_pty::{native_pty_system, Child, CommandBuilder, MasterPty, PtySize};
 use std::io::{Read, Write};
+use std::process::Command;
 use std::sync::mpsc;
 
 /// Which sandbox to summon. Multipass = strong isolation (default for real use),
@@ -36,6 +37,50 @@ impl Backend {
             Backend::Multipass => "multipass",
         }
     }
+    /// Default image/release when the user doesn't specify one.
+    pub fn default_image(self) -> &'static str {
+        match self {
+            Backend::Multipass => "24.04",
+            Backend::Docker => "ubuntu:24.04",
+            Backend::Local => "",
+        }
+    }
+}
+
+/// One-time setup before the PTY shell is spawned. Blocking — run off the UI
+/// thread (Multipass boots a real VM, ~20-30s). Idempotent: reuses an instance
+/// that already exists.
+pub fn prepare(backend: Backend, name: &str, image: &str) -> Result<()> {
+    if backend != Backend::Multipass {
+        return Ok(());
+    }
+    let exists = Command::new("multipass")
+        .args(["info", name])
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false);
+    if !exists {
+        let st = Command::new("multipass")
+            .args([
+                "launch", "--name", name, "--cpus", "1", "--memory", "1G", "--disk", "5G", image,
+            ])
+            .status()
+            .context("multipass launch (is multipass installed?)")?;
+        anyhow::ensure!(st.success(), "multipass launch failed");
+    } else {
+        let _ = Command::new("multipass").args(["start", name]).status();
+    }
+    Ok(())
+}
+
+/// Destroy ephemeral resources after stop. Multipass instance is purged;
+/// Docker uses `--rm` so the container is already gone; Local is a no-op.
+pub fn teardown(backend: Backend, name: &str) {
+    if backend == Backend::Multipass {
+        let _ = Command::new("multipass")
+            .args(["delete", name, "--purge"])
+            .status();
+    }
 }
 
 /// Build the shell command that launches the sandbox for a backend.